Many drawbacks have been identified in existing lighting control systems that can result in less than desired performance of a lighting device. These drawbacks include, but are not limited to, voltage variations among LED lighting modules that result in non-uniform light output. These voltage variations may result from the lack of uniformity in the manufacture of the LEDs used in a lighting device.
Another drawback of existing lighting control systems is the inability of the lighting circuitry to compensate for the effects of temperature changes on the LED forward voltages, such as changes required in the drive voltage caused by an increase in temperature. In this regard, existing lighting control systems do not compensate for inherent forward voltage changes as seen by an output driver over the entire operating temperature range of the lighting device.
Furthermore, as well known to those skilled in the art, the light output of an LED is inversely proportional to the junction temperature of the LED. Therefore, when the LED is first activated (i.e., cold startup), the junction temperature is low and the light output is high. As the junction temperature increases during a warm-up period (typically lasting about 30 minutes), the light output of the LED will decrease until the LED reaches a steady state condition. Once the steady state condition has been reached, the LED junction operating temperature will remain generally constant, and consequently the light output will remain generally constant during that continuous period of use.
The decay in the LED light output during the warm-up period may be as large as 20%. Therefore, the LED light output during the warm-up period is not generally uniform, and the LED light output at steady state may be significantly lower than the expected nominal light output at steady state. It is also possible that the LED light output at cold startup may exceed an upper limit light output.
The foregoing drawbacks are particularly disadvantageous where the lighting device is a surgical lighthead that requires substantially constant light output or lux readings.
The present invention addresses these and other drawbacks to provide an improved lighting control method for a lighting device.